Microbiology For COMLEX Level 1 Practice Test

The main idea here is that India ink staining of cerebrospinal fluid is particularly good for detecting encapsulated yeasts, with Cryptococcus neoformans being the classic example. Cryptococcus has a large polysaccharide capsule that resists the India ink stain, so when you look at CSF under the microscope, the capsule appears as a clear halo surrounding the yeast cell against a dark background. This capsule is a key virulence factor, helping the organism evade phagocytosis and enabling it to cause meningitis, especially in people with AIDS. Other organisms listed don’t show this capsule halo on India ink in CSF. Histoplasma capsulatum is a dimorphic fungus without a distinctive capsule seen this way; Pneumocystis jirovecii and Candida albicans aren’t identified by India ink in CSF and have different typical clinical and staining patterns. So the clear halo around a yeast in CSF on India ink strongly points to Cryptococcus neoformans, a common cause of meningitis in AIDS patients.